The present application relates to a reproducing apparatus and a reproducing method for reproducing data, recorded in the form of an interference fringe produced as a result of interference between reference light and signal light, from a holographic recording medium.
In hologram recording/reproducing technology, in particular, in hologram recording/reproducing technology for use in an optical storage, a SLM (spatial light modulator) such as a transmission-type liquid crystal panel or a DMD (Digital Micro mirror Device) is used to modulate light intensity of given signal light, for example, into a sequence of bits having a “1” level (corresponding to, for example, high intensity) or a “0” level (corresponding to, for example, low intensity).
In the light modulation, for example, as shown in FIG. 2, the SLM modulates the light intensity of signal light passing through a central part of the SLM and outputs the resultant modulated signal light such that the modulated signal light is surrounded by reference light passing through a ring-shaped peripheral part of the SLM and output therefrom. The signal light modulated according to the data to be recorded falls, together with the reference light, on a holographic recording medium so that interference between the signal light and the reference light occurs and a resultant interference fringe is recorded as data on the holographic recording medium.
In reproduction of data, only reference light is generated via the SLM, and the holographic recording medium is illuminated with the reference light output from the SLM. As a result, the reference light is diffracted by the interference fringe. An image is then formed by the diffracted light on an image sensor such as a CCD (Charge Coupled Device) or a CMOS (Complementary Oxide Semiconductor) sensor thereby reproducing a sequence of bits of data.
This method, in which signal light and reference light are provided in a coaxial manner, is called a coaxial method.
In the coaxial method, it is known to further perform a phase modulation by applying a phase mask to a real image plane of light obtained as a result of spatial light intensity modulation performed by the SLM (see, for example, Japanese Unexamined Patent Application Publication No. 2006-107663).
The phase modulation using the phase mask is performed on both the signal light and the reference light. The purpose for the phase modulation is to achieve multiplexing in recording data on a holographic recording medium, as with the technique disclosed in Japanese Unexamined Patent Application Publication No. 2006-107663. More specifically, signal light (data) recorded using reference light having a particular phase structure can be read only by illuminating the holographic recording medium with reference light having the same phase structure as that used in the recording. Therefore, if a plurality of data are respectively recorded in a multiplexed manner using reference light having difference phase structures, it is possible to selectively read desired data of the plurality of data by illuminating the holographic recording medium with reference light having the same phase structure as that used in recording the data.
The phase modulation performed on the signal light is to enhance efficiency of interference between the signal light and the reference light and to spread the spectrum of the signal light thereby to suppress a DC component, and thus to increase the recording density.
As for a phase modulation pattern to be applied to the signal light for the above purpose, for example, a binary random pattern is used. More specifically, for example, in the phase modulation pattern, a randomly selected half of pixels are set to provide a phase shift of π and the remaining half of pixels are set to provide no phase shift (phase shift=0).
As described above, use of the two-level random pattern allows an improvement in the efficiency of interference between reference light and signal light. Furthermore, the two-level random pattern causes the spectrum to be spread uniformly over a Fourier plane (the image on the medium), which can reduce the DC component included in the signal light.
The suppression of the DC component achieved by use of the phase mask is described in further detail below with reference to FIGS. 28 to 30.
FIG. 28A illustrates recorded data including random values (in an upper part of the figure), and a frequency characteristic thereof (in a lower part of the figure). In FIG. 28B, a central part of the data to be recorded shown in FIG. 28A is shown in an upper part of the figure, and a frequency characteristic thereof is shown in a lower part of the figure.
Note that actual signal light carrying data to be recorded is substantially circular in cross section, although the data to be recorded shown in the figure has a square shape for convenience of illustration. This is also true for other figures such as FIG. 29 and FIG. 30.
In the data shown in the upper parts of respective FIGS. 28A and 28B, white areas have a bit value of “1” and black areas have a bit value of “0”. In the lower parts of respective FIGS. 28A and 28B, spectral distributions indicating the frequency characteristics are represented by gray levels.
FIGS. 29A, 29B, 30A, and 30B illustrate images (SLM page images) obtained via modulation performed by the SLM in accordance with the data to be recorded, shown in FIGS. 28A and 28B, and also illustrates frequency characteristics thereof. More specifically, FIG. 29A illustrates an example of a whole SLM page image (in an upper part of the figure) produced without using the phase mask and a frequency characteristic thereof (in a lower part of the figure), and FIG. 29B illustrates central parts of the image and the frequency characteristic shown in FIG. 29A. In the upper parts of FIGS. 29A and 29B, white areas have a value of “1” and gray areas have a value of “0”.
In lower parts of FIGS. 29A and 29B, spectral distributions indicating the frequency characteristics are shown. As can be seen, the spectrum of the image produced without using a phase mask has a sharp peak in the center of the image. This means that a DC component appears in the center. Note that such a DC component recorded in a particular area can prevent other hologram pages from being recorded. In such a case, it is necessary to record data such that hologram pages are sufficiently widely spaced. This makes it difficult to achieve high recording density.
FIG. 30A illustrates an example of a whole SLM page image (in an upper part of the figure) produced using a phase mask and a frequency characteristic thereof (in a lower part of the figure), and FIG. 30B illustrates central parts of the image and the frequency characteristic shown in FIG. 30A. In the upper parts of FIGS. 30A and 30B, white areas have a value of “1” and gray areas have a value of “0”. In the lower areas of FIGS. 30A and 30B, spectral distributions indicating the frequency characteristics are represented by gray levels.
As can be seen from the figures, bits recorded as “1” in the image produced without using the phase mask are split into “+1” and “−1” in the image produced using the phase mask because the phase modulation causes the phase of each bit “1” to be shifted to either 0 or π. Note that bits “0” remain at “0” without being changed by the phase modulation.
Thus, in the case where the phase mask is used, data is recorded in three levels “0”, “+1”, and “−1”. In FIGS. 30A and 30B, the level “−1” is represented in black.
In the recording of data in three levels, use of the phase mask causes the spectrum to be spread as shown in the lower parts of FIGS. 30A and 30B. In particular, spreading of the spectrum is clearly shown in FIG. 30B. In this specific example, the peak value of the spectrum is reduced to 2.1E+4 from the peak value of 2.6E+5 obtained in the case shown in FIG. 29, that is, the peak value of the spectrum is decreased to 1/10 as a result of the spreading of the spectrum.
The spreading of the spectrum results in a reduction in the DC component, which allows more data to be recorded as a hologram page. In other words, it becomes possible to reduce a space between hologram pages, which allows an increase in recording density.